Modern MOS, in particular CMOS, integrated circuit fabrication technologies advance with ever shrinking device geometries to allow an increasing density of active elements, such as transistors. However, the shrinking of geometries puts among other things, severe limitations on allowable maximum voltages in the circuits so as to avoid for example gate oxide destructive breakdown in transistors. One way of overcoming these limitations in certain cases, is to offer so-called thick-oxide transistor devices suitable for higher voltage operation. Thus, by increasing the thickness of the gate oxide layer, a transistor can be operated at a relatively high voltage potential.
Various documents within the technical field of electronics incorporating amplifiers and/or buffers suitable for processing signals generated by electro-acoustic transducers can be found in the patent literature. Examples of such documents are WO 02/073792, US 2004/0202345, US 2005/0151589, US 2007/0009111, U.S. Pat. No. 7,149,317 and EP 1 599 067 which have all been filed by the assignee of the present invention. However, the disclosures of all of these documents all relate to electronics incorporating only thin-oxide transistors.
A less appreciated characteristic of deep sub-micron CMOS technologies is the use of oxy-nitrides to realize very thin gate oxides needed for typical thin-oxide transistors. For deep sub-micron technologies with a minimum feature size below 0.35 micron oxy-nitrides replace conventional silicon oxide as the gate oxide material of choice. A significant disadvantage of using oxy-nitrides is a dramatic increase in transistor flicker noise arising from an increased carrier trap density found in oxy-nitride gate materials. As thin-oxide transistors as a consequence of the thin gate oxide at the same time exhibit a much higher capacitance per unit gate area the noise performance of certain types of preamplifiers, in particular preamplifiers suitable for capacitive transducers, such as miniature condenser microphones and miniature electric microphones, may suffer dramatically.
Thus, an object of the present invention may be seen as to provide a MOS transistor-based preamplifier with a low-noise input stage realized in a deep sub-micron MOS technology, the MOS transistor-based low-noise input stage featuring a very low flicker noise level.